The present invention relates to a transmission control device, and to a method of transmission control implemented by said transmission control device, for an automatic transmission system for an automotive vehicle such as one for road use, and, more particularly, relates to such a transmission control device and method for an automatic transmission system incorporating a gear transmission mechanism which includes a plurality of hydraulic fluid pressure activated friction engaging mechanisms, in which an electrically actuated and controlled hydraulic fluid pressure control device selectively supplies actuating pressures to the friction engaging mechanisms of the gear transmission mechanism of the automatic transmission, so as to control the transmission between its various speed stages, i.e., gear ratios, according to selective supply of actuating electrical energy to said electrically controlled hydraulic fluid pressure control device.
Automatic transmissions for automotive vehicles which include gear transmission mechanisms including several fluid pressure activated friction engaging mechanisms such as multi plate clutches and multi plate brakes are well known in various forms. Such an automatic transmission is often conventionally controlled by a hydraulic fluid pressure control systems, which selectively controls supply of hydraulic fluid pressure to the friction engaging mechanisms, according to decisions that said hydraulic fluid pressure control system makes as to what speed stage of the gear transmission mechanism should be currently engaged, in view of and based upon the current values of various operational parameters of the vehicle, such as the road speed of the vehicle, the load upon an internal combustion engine of the vehicle, and the like.
Nowadays, however, with the rapid progress which is being attained in the field of electronic control systems, various arrangements have been proposed, in which electronic control devices make control decisions as to what speed stage of the gear transmission mechanism should be currently engaged, in view of and based upon the current values of various operational parameters of the vehicle, such as the road speed of the vehicle, the load upon an internal combustion engine of the vehicle, the currently selected transmission range, and the like. In such arrangements, an electric signal is sent by such an electronic control device to an electric to hydraulic pressure conversion device such as a solenoid valve, and this electric to hydraulic pressure conversion device is incorporated in a hydraulic fluid pressure control system which performs the actual switching of the aforesaid activating hydraulic fluid pressures to the friction engaging mechanisms. Thus the hydraulic fluid pressure control system does not actually make the decision, itself, as to which speed stage should be engaged, but instead merely follows and implements the decision made by said electronic control device.
Further, as a particular mode of operation, it is known for such an electronic control device to read a target speed stage for the automatic transmission out from a memory means such as a read only memory or ROM, according to the current values of various operational parameters of the vehicle, such as the road speed of the vehicle, the load upon the internal combustion engine of the vehicle, the currently selected transmission range, the current speed stage which is being provided by the gear transmission mechanism, etc., and then to send such an electric signal to the hydraulic fluid pressure control system as to cause said hydraulic fluid pressure control system to so switch the supply of actuating hydraulic fluid pressures to said friction engaging mechanisms of said gear transmission mechanism as to cause the actual speed stage provided by said gear transmission mechanism to be set to said target speed stage.
A particular problem that has arisen in connection with the control of an automatic transmission in such a way by an electronic control device arises as follows. If the operational parameters of the vehicle change very quickly, such as may occur in the case of quick opening or closing of the throttle valve of the engine of the vehicle by the driver thereof, or such as may occur during simultaneous shifting by the driver of the vehicle of selected transmission speed range and also changing by said driver of the amount of throttle opening of the engine of the vehicle, then it may well occur that the target speed stage for the gear transmission mechanism, as thus read out from the memory means by the electronic control device, differs by more than one speed stage from the actual or current speed stage which is being provided by said gear transmission mechanism. For example, in the case of a gear transmission mechanism which provides three forward speed stages, it may occur that the target speed stage is read out to be the third speed stage, while the current or actual speed stage is the first speed stage; or, by contrast, it may occur that the target speed stage is read out to be the first speed stage, while the current or actual speed stage is the third speed stage. Further, in the case of a gear transmission mechanism which provides more than three forward speed stages, such as in the case of one which provides four or five or more forward speed stages, other combinations of target speed stage and current speed stage which differ by more than one speed stage step are possible, and may well occur.
If it occurs that the target speed stage and the current speed stage come to differ by more than one speed stage step, the electronic control device directly controls the hyraulic fluid pressure control system to immediately switch the supply of actuating hydraulic fluid pressure to said friction engaging mechanisms of said gear transmission mechanism so as to cause the actual speed stage provided by said gear transmission mechanism to be set immediately to said target speed stage, then shifting of said gear transmission mechanism up or down by more than one speed stage step at a time will result, and this can cause undesirable consequences. In the case of an up shift by more than one speed change step at a time, particularly if the throttle opening is rather small, great transmissin shift shock and torque shock can be generated, which deteriorates the drivability of the vehicle. Further, in the case of a down shift by more than one speed change step at a time, particularly if the throttle opening is rather great, not only can great transmission shift shock and torque shock be generated, which again deteriorates the drivability of the vehicle, but also the hydraulic fluid pressure activated friction engaging mechanisms of the gear transmission mechanism, particularly those which are engaged from the disengaged condition in order to provide the low speed stage which is downshifted, may be subjected to unduly high frictional forces, which can cause damage, and can reduce the service life of the automatic transmission and of the transmission as a whole. Further, in this case there is a danger of the vehicle engine to be running at a high speed which can cause slipping of one or more of the activated friction engaging mechanisms. This can cause damage to the transmission.